In cellular networks, radio base stations must use a very accurate frequency reference for their RF transmit and receive circuitry and other components. In order to achieve the required degree of accuracy, which is typically on the order of 0.05 parts per million (ppm), this reference may require specialized hardware. Various schemes for generating an accurate frequency reference include synchronizing with an atomic clock, using a frequency derived from a dedicated backhaul connection (e.g., deriving a frequency reference from a T1, E1, or fiber optic cable that uses a Stratum-1 clock as a reference), or using a frequency reference provided by a Global Positioning System (GPS) receiver. These reference schemes are practical in larger base stations where cost sensitivity is low and a fixed line backhaul is standard.
A new type of base station providing personalized coverage has become attractive to some carriers for subscribers' homes and small offices rather than covering large districts of urban or sub-urban areas. These new base stations are known as femtocells, and are characterized by much smaller coverage areas, consumer-grade packaging and price-points, and the use of consumer internet protocol (IP) connections using various common wireline technologies. These wireline technologies, may include, but are not limited to: DSL, DOCSIS, powerline, and/or coaxial cable. The lack of a fixed line backhaul and extreme cost sensitivity of these femtocells require different synchronization schemes than larger cells use. Additionally, traditional GPS synchronization may not work with femtocells as they are typically installed indoors where a GPS receiver cannot receive a signal from the GPS satellite system that is required to provide the high accuracy frequency reference.
To meet the price point targets of femtocell base stations, traditional reference schemes cannot be implemented. Accordingly, femtocells may use a less precise oscillator which sacrifices accuracy and precision for cost. These low-cost oscillators encounter frequency drift a result of manufacturing variations or environmental factors such as temperature, humidity, or the age of the oscillator.
As a reference frequency generated by an oscillator drifts, the base station may begin to transmit outside of an allocated frequency range. This may raise an interference level (e.g., a signal-to-interference-plus-noise (SINR) level) among frequency resources which are shared by adjacent cells (e.g., base stations) in a network, affecting a service provider network's Quality of Service (QOS) as well as network service subscribers' collective Quality of Experience (QOE) within a particular portion of a data communications network. Negative effects associated with poor QOS and poor QOE (e.g., conditions largely caused by congestion and/or interference), which can be exacerbated by adding uncalibrated short-range wireless transceiver devices to a network infrastructure, may include: queuing delay, data loss, as well as blocking of new and existing network connections for certain network subscribers.
Additionally, it may take more time to arrive at an accurate reference frequency on start-up using a less precise oscillator in a short-range or femtocell base station. Thus, it would be advantageous for any calibration systems and methods to improve a base station's startup procedures in terms of synchronizing with a network frequency.
Presently, there is a need for improved systems and methods that facilitate reference frequency calibration in a low-cost base station. It would be beneficial if the calibration can be used to improve the accuracy and precision of a low-cost oscillator in order to provide a frequency generation system that is economically feasible in a femtocell base station. It would further be beneficial if the calibration improves a start-up of the base station.